Hi Roger
I have the Canon 60mm F2.8 macro which I purchased as a “standard” lens to fill a gap in my zoom range. The relatively short focal length places the front element some 9 or 10 cms from your subject, so from that perspective, longer is definitely better. Loner FL’s also produce a nicer out of focus background (bokeh?).
Extension rings are kind of like a discrete bellows unit, both are air filled mechanical devices to place the lens further from the camera body. With a bellows you get an “infinite” range of lens position but I suspect the intensity of light falls of with the inverse square law so at the fully extended end of the range, you would require exposures of maybe 1 sec plus at least?
A dedicated bellows unit that maintains all electrical connections and auto exposure metering, auto focus etc would probably run into several 100’s of $ - there was a 2nd hand Nikon unit for sale on IIS not too long ago.
I haven’t really done much macro stuff to date, but certainly the small size of the 60mm F2.8 means that it always travels with me and is easy to use.
If you get a set of bellows and a macro flash then you will have additional bulky items to carry around as well as a more complex set up than just clipping in a dedicated macro lens.
Here is the Novoflex site for bellows:
http://www.novoflex.com/english/html/products.htm
Cheers
Dennis